Various techniques for preventing a leak sound from being heard utilizing a masking effect have been proposed. The masking effect is a phenomenon that when two kinds of sounds travel through the same space, one sound (masking sound) serves as an obstacle to hearing of the other sound (target sound) by a listener in the space. Many of the techniques of this kind are such that a masking sound is emitted toward a space that is adjacent to, via a wall or a screen, a space where a speaker as a source of a target sound exists.
Patent document 1 discloses a technique of generating a masking sound for preventing a human voice as a target sound from being heard by processing its sound waveform. In a masking method disclosed in the same document, a sound signal representing a human voice is divided into plural segments in intervals each of which corresponds to one phoneme. A sound signal obtained by rearranging the positions of the plural divisional segments randomly is reproduced as a masking sound. The meaning of a sound obtained by the technique cannot be understood though it seems like a human voice. The use, as a masking sound, of such a sound can provide a higher masking effect than in the case of using a sound having a wide spectrum such as an environment sound.